Fluorescent materials comprising silicate, phosphate, aluminate, or sulfide as a host material and containing a transition metal or rare earth metal as a luminescent center are widely known.
Meanwhile, white light-emitting diodes (white LEDs) that emit visible light when excited by a high-energy excitation source such as ultraviolet or blue light have attracted attention, and their development is underway.
However, the above-mentioned conventional fluorescent materials have a problem that their brightness decreases when they are exposed to the excitation source.
As fluorescent materials unsusceptible to decrease in brightness, nitride and oxynitride fluorescent materials, which have stable crystallographic structure and allow shifting the excitation or emission light toward the longer wavelength side, are receiving attention.
As a nitride or oxynitride fluorescent material, an α-Sialon doped with a specific rare earth element is known to have useful fluorescent property, hence its application to a white LED is being investigated (see Patent References 1 to 5 and Nonpatent Reference 1.)
In addition, Ca2(Si, Al)5N8, CaSiAlN3, and β-Sialon doped with a rare earth element, have been found to have similar fluorescent property (see Patent References 6 to 8 and Nonpatent References 2 and 3.)
Other fluorescent materials proposed include those that use nitrides or oxynitrides, such as aluminum nitride, magnesium silicon nitride, calcium silicon nitride, barium silicon nitride, gallium nitride, and zinc silicon nitride, as a host material.
A β-Sialon is a solid solution of β silicon nitride in which Si and N sites are substituted with Al and O dissolving in solid solution, respectively. Since there are two formula weight atoms in a unit cell, Si6-zAlzOzN8-z is used for a general formula of the β-Sialon. Composition Z in this formula falls within the 0 to 4.2 range, implying a wide solution range, whereas the (Si, Al)/(N, O) molar ratio must be maintained at 3/4. Consequently, silicon oxide and aluminum nitride, or aluminum oxide and aluminum nitride, are generally used, in addition to silicon nitride, as raw materials, and heated to obtain a β-Sialon.
When Eu ions are dissolved thoroughly into a β-Sialon crystal structure, they are excited by ultraviolet or blue light, and emits green or yellow light having wavelengths of 500 to 550 nm.
Unlike monochromatic light, white light of a white LED is produced based on multiple color combinations. As disclosed in Patent References 9 and 10, conventional white LEDs use combinations of an ultraviolet or blue LED and a fluorescent material that generates visible light using the light from that LED as an excitation source. Consequently, if the intensity of light emitted from the fluorescent material is low, that of the white light emitted from the LED also decreases.
Unlike colored light used for signal light or identification lamps, white light is generally used for illuminating objects. Consequently, when a white LED is used as a backlight of display device such a liquid crystal display device or various general illuminators for indoor and outdoor application, obtaining white light having sufficient intensity is essential.
Patent Reference 1: Japanese Patent Laid Open Application No. JP2002-363554A
Patent Reference 2: Japanese Patent Laid Open Application No. JP2003-336059A
Patent Reference 3: Japanese Patent Laid Open Application No. JP2003-124527A
Patent Reference 4: Japanese Patent Laid Open Application No. JP2003-206481A
Patent Reference 5: Japanese Patent Laid Open Application No. JP2004-186278A
Patent Reference 6: Japanese Patent Laid Open Application No. JP2004-244560A
Patent Reference 7: Japanese Patent Laid Open Application No. JP2005-255895A
Nonpatent Reference 1: J. W. H. van Krebel, “On new rare earth doped M-Si—Al—O—N materials,” TU Eindhoven, The Netherlands, p. 145-161 (1998)
Nonpatent Reference 2: Extended Abstracts of the 65th Meeting, Japan Society of Applied Physics, (September 2004, Tohoku Gakuin University), No. 3, pp. 1282-1284
Nonpatent Reference 3: Extended Abstracts of the 52nd Meeting, Japan Society of Applied Physics and Related Societies, (March 2005, Saitama University) No. 3, p. 1615
Patent Reference 8: Japanese Patent Laid Open Application No. JP2005-255895A
Patent Reference 9: Japanese Patent Laid Open Application No. JP1993-152609A
Patent Reference 10: Japanese Patent Laid Open Application No. JP1995-99345A
Patent Reference 11: Japanese Patent 2927279